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Lecture 7

NATS 1940 Lecture Notes - Lecture 7: Gamete, Allele, Antirrhinum


Department
Natural Science
Course Code
NATS 1940
Professor
Mark Vicari
Lecture
7

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BIODIVERSITY AND CONSERVATION BIOLOGY
LECTURE 7
SEPTEMBER 28, 2017
MENDELS MODEL OF HEREDITY
NON-MENDELIAN INHERITANCE
Example true breeding= homozygous
Medel’s Next Question:
How are pairs of genes inherited?
o Each pea plant has two alleles for every character
Ex, ppYYrr… etc
Flower color seed shape
Seed color
Each gamete has one allele per character
e.g. pYr…etc.
Hypothesis
1. Dominant and recessive alleles stay together in gametes
PpYy PY and py gametes only
2. Dominant and recessive alleles segregate independently
PpYy PY, Py, pY, py gametes possible
Hypothesis 1:
PY py
PY
Py
PPYY
PpYy
PpYy
ppyy
3 puprle/ yellow: 1 white/green
3:1

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Hypothesis 2:
PY Py pY py
PY
Py
pY
py
Crossed true-breeding lines
PPYY * ppyy
F1 = 100% PpYy
“dihybrids”
F2 = 9:3:3:1
purple/yellow: purple/green: white/yellow: white/green
Supports hypothesis 2
Mendel’s Second Law (law of independent assortment)
Each pair of alleles segregated independently of other pairs of alleles during gamete
formation
o NOTE: we now know the “law” does not apply in all situations!
Question
Recall: In pea plants,
purple flowers (P) are dominant over white (p)
yellow seeds (Y) are dominant over green (y)
A true-breeding purple-flowered, green-seeded pea plant is crossed with a white-flowered,
non-true-breeding yellow-seeded pea plant. What is the expected phenotypic ratio among the
offspring?
1st step: write out genotypes
o true breeding means homozygous
o hence, PPyy X ppYy
2nd step: write out gametes
o Py pY
o Py py
PPYY
PPYp
PpYY
PpYy
PPYy
PPyy
PpYy
Ppyy
PpYY
PpYy
ppYY
ppYy
PpYy
Ppyy
ppYy
ppyy
9 that start with
P and Y
3 that start with
P and 2 small y
3 that has 2
small p and at
least 1 big Y
1 that has 2
small p and 2
small y

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o Py pY
o Py py
3rd step: fill in Punnett square
pY py pY py
Py
Py
Py
Py
Purple, yellow: 8
Purple, green: 8 1:1
White, yellow:0
White, green: 0
Non-Mendelian Inheritance
“Mendelian” characters
-Controlled by a single gene with two alleles, one dominant one recessive
In “Non-Mendelian” characters, inheritance of traits does not follow this simple
pattern. Some non-mendelian factors
1. Epistasis
The modification of one gene’s expression by another
Example for Epistasis: Fur color in mice
BB
Bb -black or white
bb brown or white
2nd gene:
C- pigment deposited in fur
c-pigment not deposited in fur
BBCc= black
BBcc= white
Another Question:
6. What will be the phenotype ratios of the offspring of a dihybrid mouse (BbCc) and a
white mouse with two brown parents?
Ppyy
PpYy
Ppyy
b
l
a
c
k
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